An endoscope for medical use, which is for imaging the inside of a living body cavity, is known. A flexible tube is a main part of an insert section of the endoscope, which is inserted into the inside of a living body cavity, and is constituted of a helical coil made of a metal ribbon wound helically, and a covering layer made of resin such as a urethane resin on the periphery of a tubular net. The covering layer is over-molded by extrusion, on the periphery of a flexible tube material formed by covering the helical coil with a tubular net. The flexible tube is expected to have high flexibility (to be soft) at a distal end for better insertability into the living body cavity, and low flexibility (hard) at a proximal end for better operatability.
For this reason, it is known that a covering layer is two-layer molded to have an inner layer and an outer layer whose hardness are different from each other, and that, though the outside diameter of the covering layer is constant throughout the whole length, the flexibility of flexible tube in the axial direction is changed (a difference is made in the hardness) by varying a thickness ratio between the inner layer and the outer layer in the axial direction of flexible tube (patent documents 1 and 2).
As one of the methods to two-layer mold the covering layer, the two-layer simultaneous molding method, in which the inner layer and the outer layer constituting the covering layer are molded at the same time, are known. In the two-layer simultaneous molding method, two kinds of resins are heated at a respective appropriate temperature in two extrusion sections provided in an extrusion machine, to be in a molten condition. And two kinds of resins in the molten condition join before being supplied into a molding die in which a molding passage where flexible tube materials from each extrusion section pass is formed, and are supplied to the molding die in the state that two kinds of resins are overlapped. In this way, the covering layer of two-layers constitution covers the periphery of the flexible tube material.